Image forming apparatus, image forming system, method of calibrating image detection unit, and post-processing apparatus

ABSTRACT

An image forming apparatus that includes an image forming unit that forms an image on a transfer medium, a conveying unit that conveys the transfer medium, a first image detection unit that detects the image formed by the image forming unit on the transfer medium; and a control unit that controls the forming of the image and the conveying of the transfer medium, and receives detection results from the first image detection unit and a second image detection unit, wherein the control unit determines whether the image detected by the first image detection unit has image quality higher than or equal to predetermined quality and calibrates the first image detection unit based on the detection result from the first image detection unit and the detection result from the second image detection unit for the image having the image quality higher than or equal to predetermined quality.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/934,901, filed on Nov. 6, 2016, which claims priority to and thebenefit of Japanese Patent Application No. 2014243386, filed on Dec. 1,2014, all of which are incorporated herein by reference in theirentirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to an image forming apparatus, an imageforming system, a method of calibrating an image detection unit, and apost-processing apparatus for detecting an image formed on a transfermedium.

Description of the Related Art

In an image forming apparatus such as a copying machine, a fax machine,a printer, and a multifunction printer, an image is formed on a transferpaper by an image forming unit based on image data. Such an imageforming process is performed according to an image forming condition(position, color, density and the like for the image forming) based onthe image data. As the image forming apparatus is operated, the imageforming condition can change temporarily or gradually, so quality ismaintained through various kinds of an automatic adjustment function andthe like.

For example, a patch image or the like is formed on a transfer paper atintervals of predetermined time of the operation of the image formingapparatus or predetermined number of printed sheets. The patch image isread by a line sensor or the like, and then the state of the image andthe image forming condition are compared to determine if the image isformed properly. If the image is not properly formed according to theimage forming condition and there is misalignment, calibration or thelike is made on the image forming condition to maintain the quality ofthe image.

For example, in Japanese Patent Laid-Open No. 2007-30340, an image isread by a sensor apparatus including a colorimeter or a densitometer,and then density of a toner image formed by an image forming unit isadjusted according to the read result. Further, in Japanese PatentLaid-Open No. 2007-30340, if there is a stain on a part of the patch, apatch image is formed and read for an area excluding the area with thestain so as to be able to adjust output density.

For reading of an image, a line sensor using a CCD or a CIS as an imagecapturing element is widely used. The line sensor is suitable forhigh-speed reading and can read a fixed transfer paper at conveyingspeed of a fixing part, and then inspection can be made for colorreproducibility, print positioning and the like. However, the linesensor is not suitable for color reproducibility, and therefore it ispreferable that the line sensor is subject to periodic calibration usinga spectral colorimeter for which colorimetric accuracy is ensured.

However, when calibration of the line sensor is performed with aspectral colorimeter in a configuration where the spectral colorimeterand the line sensor are provided on the same conveying path, a patch onthe same chart needs be read. The spectral colorimeter can only measurea color as an average value in a spot area of a patch on the chart, andcannot determine if the patch is properly printed. The problem is thatthe spectral colorimeter cannot accurately measure colors of a patchwhen there is uneven density, a stain, a scratch or the like on thechart and therefore cannot properly perform calibration of the linesensor.

The present invention is made in view of the background above. Oneobject of the present invention is to read an image and checks whetherthere is uneven density, a stain, a scratch and the like in order todetermine print quality by a first image detection unit, and read by asecond image detection unit the image determined as its printing qualitybeing OK in order to calibrate the first image detection unit.

SUMMARY OF THE INVENTION

To achieve at least one of the abovementioned objects, an image formingapparatus reflecting one aspect of the present invention comprises animage forming unit that forms an image on a transfer medium, a conveyingunit that conveys the transfer medium through a conveying path, a firstimage detection unit that detects the image formed by the image formingunit on the transfer medium conveyed by the conveying unit, a secondimage detection unit that detects the image formed by the image formingunit on the transfer medium conveyed by the conveying unit, and isdifferent from the first image detection unit, and a control unit thatcontrols the forming of the image and the conveying of the transfermedium, and receives detection results from the first image detectionunit and the second image detection unit. The control unit has adetection-unit calibration mode for calibrating the first imagedetection unit with use of the detection result of the image from thesecond image detection unit. The control unit determines, on thedetection-unit calibration mode, whether the image detected by the firstimage detection unit has image quality higher than or equal topredetermined quality, and the control unit calibrates the first imagedetection unit based on the detection result from the first imagedetection unit and the detection result from the second image detectionunit for the image having the image quality higher than or equal topredetermined quality.

In the image forming apparatus according to the abovementioned aspect,it is preferable that further comprises the second image detection unitthat detects the image formed by the image forming unit on the transfermedium conveyed by the conveying unit.

In the image forming apparatus according to the abovementioned aspect,it is preferable that the control unit receives a detection result fromthe second image detection unit that is equipped in another apparatusand that detects an image of the transfer medium.

In the image forming apparatus according to the abovementioned aspect,it is preferable that the second image detection unit detects an imageon the transfer medium on which the image is formed by the image formingunit and which is conveyed by the conveying unit.

In the image forming apparatus according to the abovementioned aspect,it is preferable that the control unit determines whether an imagedetected by the first image detection unit has image quality higher thanor equal to predetermined quality based on the entire image or a part ofthe image having been read.

In the image forming apparatus according to the abovementioned aspect,it is preferable that, when the image detected by the first imagedetection unit does not have image quality higher than or equal to thepredetermined quality, the control unit determines which of the firstimage detection unit and the transfer medium has caused the result thatthe image does not have image quality higher than or equal to thepredetermined quality, and, when the first image detection unit hascaused the result, the control unit uses the detection result of theimage from the second image detection unit.

In the image forming apparatus according to the abovementioned aspect,it is preferable that the control unit determines whether the detectionresult detected by the first image detection unit can be processed to beusable for the calibration when the image detected by the first imagedetection unit does not have image quality higher than or equal to thepredetermined quality.

In the image forming apparatus according to the abovementioned aspect,it is preferable that the control unit determines whether thecalibration for the first image detection unit can be achieved ataccuracy higher than or equal to predetermined accuracy when only a partof the image detected by the first image detection unit is used, thepart having the image quality higher than or equal to the predeterminedquality.

In the image forming apparatus according to the abovementioned aspect,it is preferable that, when the image detected by the first imagedetection unit has a defective area that does not have image qualityhigher than or equal to the predetermined quality, the control unit usesthe detection result of the image from the second image detection unitbased on a part of the image excluding the defective area.

In the image forming apparatus according to the abovementioned aspect,it is preferable that the control unit detects a position of an imagebased on the detection result from the first image detection unit anduses the detection result of the image from the second image detectionunit at the detected position of the image.

In the image forming apparatus according to the abovementioned aspect,it is preferable that the control unit perform control so that based ona position of the image, the second image detection unit or the transfermedium or both of them are moved for the detection of the image.

In the image forming apparatus according to the abovementioned aspect,it is preferable that the control unit calculates a calibration valuefor the first image detection unit using the detection result from thesecond image detection unit, for the image detected by the first imagedetection unit and the second image detection unit, when the imagedetected by the first image detection unit has image quality higher thanor equal to the predetermined quality.

In the image forming apparatus according to the abovementioned aspect,it is preferable that the control unit determines whether each patchimage is usable or not when a plurality of patch images are formed onthe transfer medium and only a part of the image is used for thedetection of the image.

In the image forming apparatus according to the abovementioned aspect,it is preferable that the control unit forms a plurality of patch imageshaving the same density and uses the detection result from the secondimage detection unit only for those patch images whose images detectedby the first image detection unit have the image quality higher than orequal to the predetermined quality among the patch images having thesame density.

In the image forming apparatus according to the abovementioned aspect,it is preferable that the image forming apparatus includes a reversepath that receives the transfer medium and reverses the front and theback of the transfer medium at the downstream side of the image formingunit, and returns back the transfer medium at an upper stream side ofthe image forming unit. When at least the image detected by the firstimage detection unit does not have image quality higher than or equal tothe predetermined quality, the control unit detects an image formed onthe back side at the first image detection unit and the second imagedetection unit, and determines whether the image detected by the firstimage detection unit has image quality higher than or equal to thepredetermined quality, and the control unit performs calibration of thefirst image detection unit for the image having the image quality higherthan or equal to the predetermined quality, based on the detectionresult from the first image detection unit and the detection result fromthe second image detection unit.

In the image forming apparatus according to the abovementioned aspect,it is preferable that, after the calibration of the first imagedetection unit, the control unit determines validity of the calibrationresult by reading the image on the transfer medium with the first imagedetection unit and the second image detection unit.

In the image forming apparatus according to the abovementioned aspect,it is preferable that the first image detection unit is a line sensor.

In the image forming apparatus according to the abovementioned aspect,it is preferable that the second image detection unit is a spectralcolorimeter

To achieve at least one of the abovementioned objects, an image formingsystem reflecting one aspect of the present invention comprises acontrol unit that receives detection results from a first imagedetection unit that detects an image on a transfer medium and a secondimage detection unit that detects the image of the transfer medium andis different from the first image detection unit, wherein the controlunit has a detection-unit calibration mode for calibrating the firstimage detection unit with use of the detection result of the image fromthe second image detection unit; the control unit determines, on thedetection-unit calibration mode, whether the image detected by the firstimage detection unit has image quality higher than or equal topredetermined quality, and the control unit performs control processesfor calibrating, when the image quality is higher than or equal to thepredetermined quality, the first image detection unit based on thedetection result from the first image detection unit and the detectionresult from the second image detection unit for the image having theimage quality higher than or equal to the predetermined quality.

To achieve at least one of the abovementioned objects, a method ofcalibrating an image detection unit reflecting one aspect of the presentinvention comprises a reception step for receiving detection results ofan image on a transfer medium being conveyed detected by a first imagedetection unit and a second image detection unit, and a calibration stepfor determining whether an image detected by the first image detectionunit has image quality higher than or equal to predetermined quality andcalibrating, when the image quality is higher than or equal to thepredetermined quality, the first image detection unit based on thedetection result from the first image detection unit and the detectionresult from the second image detection unit.

In the method of calibrating an image detection unit according to theabovementioned aspect, it is preferable that the method includes a firstimage detection step for detecting the image on the conveyed transfermedium with the first image detection unit, and a second image detectionstep for detecting the image on the conveyed transfer medium with thesecond image detection unit.

To achieve at least one of the abovementioned objects, a post-processingapparatus reflecting one aspect of the present invention comprises aconveying unit that conveys a transfer medium on which an image isformed through a conveying path a second image detection unit thatdetects the image on the transfer medium being conveyed by the conveyingunit, which is different from a first image detection unit, a controlunit that receives detections results from the first image detectionunit and the second image detection unit, wherein the control unitdetermines whether the image detected by the first image detection unithas image quality higher than or equal to predetermined quality, and thecontrol unit performs control processes for calibrating, when the imagequality is higher than or equal to the predetermined quality, the firstimage detection unit based on the detection result from the first imagedetection unit and the detection result from the second image detectionunit for the image having the image quality higher than or equal to thepredetermined quality.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a mechanical scheme of an image forming system according toan embodiment of the present invention;

FIG. 2 shows a plan view illustrating an arrangement relation of a firstimage detection unit and a second image detection unit thereof;

FIG. 3 shows a function block diagram of a control unit thereof;

FIG. 4 shows a flowchart of an overall scheme of printing andcalibration processes thereof;

FIG. 5 shows a flowchart of procedure for a printing operation thereof;

FIG. 6 shows a flowchart of procedure for a detection-unit calibrationoperation thereof;

FIG. 7 shows a flowchart of procedure for line sensor control thereof;

FIG. 8 shows a flowchart of procedure for colorimeter control thereof;

FIG. 9 shows a flowchart of procedure for patch quality determinationthereof;

FIG. 10 shows a flowchart of procedure for calculating a calibrationvalue thereof;

FIG. 11 shows a normal printing display used for detection-unitcalibration thereof;

FIG. 12 shows a calibration chart display used for detection-unitcalibration thereof;

FIG. 13 shows a transfer image read from the calibration chart displayused for detection-unit calibration thereof;

FIG. 14 shows an image forming system including a colorimeter movingunit according to another embodiment of the present invention;

FIG. 15 shows a plan view illustrating an arrangement relation of afirst image detection unit and a second image detection unit thereof;

FIG. 16 shows a calibration chart display used for detection-unitcalibration thereof;

FIG. 17 shows a transfer image read from the calibration chart displayused for detection-unit calibration thereof;

FIG. 18 shows an image forming system according to further anotherembodiment of the present invention;

FIG. 19 shows a flowchart of procedure for a detection-unit calibrationoperation thereof;

FIG. 20 shows a flowchart of procedure for a detection-unit calibrationoperation according to further another embodiment of the presentinvention; and

FIG. 21 shows a flowchart of procedure for patch quality determinationthereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention are described below.

FIG. 1 shows an image forming system 1. The image forming system 1 canbe consisted of only an image forming apparatus 10 or can be consistedof an image forming apparatus 10 being connected to other apparatuses.

The image forming apparatus 10 includes an image forming unit 110 thatforms an image on a transfer paper. The image forming unit 110 hasphotoreceptors 111C, 111M, 111Y, and 111K (collectively referred to asphotoreceptors 111 hereinafter) represented for each color (cyan,magenta, yellow, black, and the like). At circumferences of eachphotoreceptor 111C, 111M, 111Y and 111K, a development unit and awriting unit (not shown) including a charger and LD for each color areprovided. On the surface of the photoreceptor 111 charged by thecharger, image exposure is carried out by the writing unit based onimage information of draft data recorded in an image memory and thelike. A latent image is thus formed on the surface of the photoreceptor111. The latent image is developed by the development unit to be a tonerimage. The toner image is transferred on a middle transfer belt 112, andthen transferred from the middle transfer belt 112 to a transfer paperconveyed by a secondary transfer roller 113 along a conveying path 115.The conveying path 115 is extended toward a paper discharging path 115Boutside the apparatus.

The transfer paper on which the image is transferred is fixed withheating by a fixing device 114 and conveyed through the conveying path115. An image forming apparatus of the present invention can be appliedto a monochrome machine and can be use other special colors (e.g., clearcolor) other than the colors described above.

A reverse conveying path 116 is diverged from a downstream side of thefixing device 114 of the conveying path 115 to reverse the transferpaper on which an image is formed and circulate the transfer paper backto an upper stream side of the image forming unit 110.

The image forming apparatus 10 includes, at the lower part thereof, apaper feeding tray 120 that stores sheets of transfer paper. Theconveying path 115 leads from the paper feeding tray 120 to the imageforming unit 110, and then from the image forming unit 110 to the paperdischarging path 115B. The transfer paper corresponds to the transfermedium of the present invention. The transfer medium is not limited topaper and can be other mediums such as OHP and cloth.

A transfer paper stored in the paper feeding tray 120 is forwarded tothe conveying path 115 and conveyed to the secondary transfer roller113. At the secondary transfer roller 113, a color image or a monochromeimage on the middle transfer belt 112 is transferred on the transferpaper. On the transfer paper on which the image is transferred, a tonerimage thereon is fixed by adding heat and pressure at the fixing device114.

Further, a line sensor 130 that reads an image on the upper surface ofthe transfer paper is provided at the upper side of the conveying path115 and at the downstream side of the point where the reverse conveyingpath 116 is diverged from the conveying path 115. The line sensor 130can adopt imaging elements such as CCD and CIS, and it is preferablethat the line sensor 130 is configured to have longer length than thewidth of transfer paper conveyed along the conveying path 115. The linesensor 130 corresponds to a first detection unit of the presentinvention. The first image detection unit is not limited to a particulartype as long as an image on the transfer paper can be detected. Further,the first detection unit can be configured to read an image by a surfacelevel or by integration of a dot level. Further, a line-sensor transferpaper detection sensor 130A is provided at the starting position for theline sensor 130 to read an image on the conveying path 115. Theline-sensor transfer paper detection sensor 130A can use an opticalsensor and the like to detect a paper.

A spectral colorimeter 131 is provided on the conveying path 115 at thedownstream side of the line sensor 130 to read colors of the image onthe transfer paper conveyed through the conveying path 115. The spectralcolorimeter 131 emits illuminating light onto the transfer paper andreceives light with a sensor reflected on the image of the transferpaper. The spectral colorimeter 131 quantifies colors with numbers bydispersing received light and expressing components of the light. Thespectral colorimeter 131 corresponds to a second image detection unit ofthe present invention. A colorimetry-object bearing transfer paperdetection sensor 131A is provided at the starting position for thespectral colorimeter 131 to read an image on the conveying path 115. Thecolorimetry-object bearing transfer paper detection sensor 131A can usean optical sensor and the like to detect a paper.

A colorimetry object conveying roller 117 is provided at the downstreamside of the line sensor 130 and the upper stream side of the spectralcolorimeter 131. The colorimetry object conveying roller 117 can lowerconveying speed at which the transfer paper is conveyed to thedownstream side, and the conveying path past the colorimetry objectconveying roller 117 is the colorimetry object conveying path 117A.

Further, at the downstream side of the spectral colorimeter 131, acirculating path 118 is diverged before the paper discharging path 115B.The other end of the circulating path 118 is converged into theconveying path 115 at the upper stream side of the line sensor 130. Theconveying path 115 can switch to the path leading to the paperdischarging path 115B or to the path leading to the circulating path118. When the transfer paper is conveyed to the circulating path 118,the transfer paper is returned back to the conveying path 115 again, andan image can be detected by the line sensor 130 and the spectralcolorimeter 131.

In this embodiment, the line sensor 130 is used as the first imagedetection unit, and the spectral colorimeter 131 is used as the secondimage detection unit, but types of the first image detection unit andthe second image detection unit are not limited to these configuration.For example, when the first image detection unit lacks a specificdetection feature, the second image detection unit can be realized witha unit having superior detection function as to this detection feature.

Further, an operation unit 121 is provided at the upper side of theimage forming apparatus 10 to display information and accept useroperations. The unit for operation and the unit for display can beseparately provided.

FIG. 2 shows a plan view of an arrangement state of the line sensor 130and the spectral colorimeter 131. Crop marks T0 to T3 and patches P1 toP4 are formed on a transfer paper S. A background board 115A ispositioned on the conveying path 115. The area measured by the spectralcolorimeter 131 is limited compared to that of the line sensor 130.Therefore, it is preferable that the patch image is formed along theposition of the main scanning direction of the spectral colorimeter 131.

Further, the image forming system 1 includes a control unit 100. Thecontrol unit 100 includes a CPU, a program for operating the CPU, a ROMfor storing the program, a RAM for workspace or the like, and anonvolatile memory for storing operation parameters and the like. Thecontrol unit 100 controls the entire image forming system and receivesreading results from the image forming unit 110, the conveying path 115,the line sensor 130, and the spectral colorimeter 131. The control unitof the present invention is not limited to be equipped in the imageforming apparatus 10 but can be equipped in a server or apost-processing apparatus.

In this embodiment, the description above illustrates a case where theimage forming apparatus 10 includes the first image detection unit andthe second image detection unit therein. However, the image formingapparatus 10 can include only the first image detection unit and apost-processing apparatus can include the second image detection unit.Further, the image forming apparatus may not include the first imagedetection unit and the second image detection unit, and thepost-processing apparatus can include both the first image detectionunit and the second image detection unit. Further, the first imagedetection unit and the second image detection unit can be included indifferent post-processing apparatuses. When the post-processingapparatus includes the first image detection unit or both the firstimage detection unit and the second image detection unit, each apparatuscan be connected online or can be at the offline state. In case of theoffline state, data can be prepared in a manner such that controlcontents for calibration can be displayed in the post-processingapparatus, and control contents can be stored in a removable storageunit and then the data for calibration contents can be transferred tothe apparatus to which calibration needs to be made. These operationsare included for the control processes for calibration.

FIG. 3 shows a function diagram of the control unit 100 and otherperipherals.

A state management unit 101 controls the entire image forming system 1and manages states of the entire image forming system 1. A storage unit102 is connected to the state management unit 101. The storage unit 102includes a ROM for storing a program or the like therein, a RAM forworkspace, and a nonvolatile memory such as flash memory and HDD forstoring data in a nonvolatile way. The nonvolatile memory storesoperation parameters of the image forming system, a threshold value formaking a determination process on quality of the image read, an imageforming calibration condition for calibrating the image, adetection-unit calibration condition for calibrating an image detectionunit, and patch image data and the like in a readable and writablestate.

A printing control unit 103 is connected to the state management unit101. An image forming unit 110 is connected to the printing control unit103 in a manner such that the image forming unit can be controlled. Theprinting control unit 103 controls the image forming unit 110 to form animage on a transfer paper.

A paper feeding control unit 104 is connected to the state managementunit 101. The paper feeding control unit 104 controls the paper feedingtray 120 to feed a transfer paper to the conveying path 115. Further, aconveying control unit 106 is connected to the state management unit101. The conveying control unit 106 controls the conveying unit forconveying the transfer paper fed from the paper feeding tray 120. Theconveying unit is configured with the conveying path 115, the reverseconveying path 116, a conveying roller on the conveying path, acolorimetry object conveying roller 117 that is described below, and amotor for driving these.

A line sensor control unit 107 is connected to the state management unit101. The line sensor control unit 107 controls operations of the linesensor 130, and receives image data on the transfer paper that is readby the line sensor 130, and then sends the image data to the statemanagement unit 101.

A colorimeter control unit 108 is connected to the state management unit101 in a manner such that the colorimeter control unit 108 can becontrolled. The colorimeter control unit 108 controls operations of thespectral colorimeter 131 and obtains a result of reading an image at thespectral colorimeter 131, and then sends the result to the statemanagement unit 101.

A calibration unit 109 is connected to the state management unit 101 ina manner such that the calibration unit 109 can be controlled. Thecalibration unit 109 receives from the state management unit 101 theresult of reading the image at the line sensor 130 and the result ofreading the draft at the spectral colorimeter 131.

On an image forming calibration mode, the calibration unit 109 obtainsthe image reading result at the line sensor 130, compares the readingresult with image forming conditions, and performs calibrations byamending image forming if needed. At this process, when the line sensor130 has been calibrated based on the reading result from the spectralcolorimeter 131, the calibrated result is used.

Further, on a detection-unit calibration mode, the calibration unit 109obtains the reading results on the same image on the same transfer paperfrom the line sensor 130 and the spectral colorimeter 131 and thencalibrates the line sensor 130 as to color reproducibility if needed.

Further, a quality determining unit 140 is connected to the statemanagement unit 101 in a manner such that the quality determining unit140 can be controlled. The quality determining unit 140 obtains theimage read by the line sensor 130 and then detects positions of theimage and performs a determining process on quality of the image read.Based on the determination on the image quality, the quality determiningunit 140 can determine whether the image has predetermined quality. Thelevels for the predetermined quality are stored in the storage unit 102.

In the function blocks of FIG. 3, the control unit 100 includes blocksper each function except the image forming unit 110, the line sensor130, and the spectral colorimeter 131.

The following describes basic operations of the image forming apparatus10.

When a user performs an operation for starting printing in the imageforming apparatus 10 via the operation unit 121 or external devices,image data is obtained from a draft reading unit or the like (notshown). When calibration of the image forming or calibration of thedetection unit starts, data of the patch image is obtained from thestorage unit 102.

Based on the generated image data, the image forming unit 110 forms, viacontrols of the printing control unit 103, an electrostatic latent imageon the surface of the photoreceptor 111 where image exposure means areevenly charged. The formed electrostatic latent image is developed intoa toner image by a development unit. Then, the toner image formed on thesurface of the photoreceptor 111 is transferred on a middle transferbelt 112 (first transfer). The way of obtaining image data is notlimited to the description above and the image data can be obtained vianetwork.

Upon the operation for starting printing, the transfer paper, which isstacked and stored in the paper feeding tray 120, is forwarded (fed) tothe conveying path 115 one by one via the control by the paper feedingcontrol unit 104. The transfer paper is then conveyed through theconveying path 115 via the control by the conveying control unit 106.The transfer paper is conveyed to the contact position with thesecondary transfer roller 113 in synchronization with the toner image onthe middle transfer belt 112.

At the position of the secondary transfer roller 113, the toner image onthe middle transfer belt 112 is transferred on the transfer paper(secondary transfer). In this secondary transfer, high positive transfervoltage is applied to the secondary transfer roller 113 to attract thenegatively charged toner image from the middle transfer belt 112 ontothe transfer paper in an electrostatic manner to process the transfer.The toner image that is transferred on the transfer paper is positionedat the upper surface side along the conveying path 115 and then fixedonto the transfer paper S through heating and melting at the fixingdevice 114. After that, in case of a single-side printing mode, thetransfer paper is ejected to the downstream side.

On a double-side printing mode, the conveying path is switched, and thetransfer paper on which the image is formed is conveyed from theconveying path 115 to the reverse conveying path 116, where the frontand the back of the transfer paper are reversed and the transfer paperis circulated to the conveying path 115 of the upper stream side of thesecondary transfer roller 113. The transfer paper S that is circulatedto the conveying path 115 has its front and back reversed, andtherefore, in the same manner as above, the image transferred on themiddle transfer belt 112 is transferred onto the back surface side ofthe image by the secondary transfer roller 113. After that, the image isfixed at the transfer paper by the fixing device 114 and then conveyedto the downstream side.

On the image forming calibration mode or the detection-unit calibrationmode, the image on the transfer paper conveyed on the conveying path 115is read by the line sensor 130. At this process, the line sensor 130 isoperated through the control by the line sensor control unit 107 to readthe image on the transfer paper being conveyed along the conveying path115. The image read is sent to the control unit 100 and used for thecalibration for the image forming or the calibration for the line sensor130.

When the image is read by the spectral colorimeter 131 at the downstreamside of the line sensor 130, the conveying control unit 106 can controlthe colorimetry object conveying roller 117 to adjust the conveyingspeed of the transfer paper lower than the normal conveying speed inimage forming. The conveying speed can be configured in a manner suchthat an operator can set the conveying speed through the operation unit121. As described above, the storage unit 102 can store the transferpaper conveying speed after changed or the normal transfer paperconveying speed.

The colorimeter control unit 108 controls the operation of the spectralcolorimeter 131 to read the image on the transfer paper being conveyedalong the conveying path 115. The reading result is sent to the controlunit 100 and used for the calibration of the line sensor 130.

Operation timings for the image forming calibration or thedetection-unit calibration can be previously set according to theoperation time of the apparatus or the number of printing sheets in amanner such that the calibrations are automatically executed at thepreviously set timings. Further, there can be configuration where theoperator can provide an instruction to execute the calibrations via theoperation unit 121.

The following describes the overall procedure including the printingoperation and the detection-unit calibration with reference to theflowchart shown in FIG. 4. The following procedure is performed underthe control by the control unit 100.

s100: Detection Calibration?

The process is started, and it is determined whether the condition fordetection calibration is met (Step s100). Whether detection calibrationis needed or not can be determined according to predeterminedconditions.

The detection calibration is, for example, performed based on theconditions shown in Table 1. The settings No. 1 and 2 are conditions setby a user via the operation unit while the settings No. 3 and 4 areconditions occurring when the state of the apparatus changes, forexample, after maintenance on parts. It can be determined that thecondition for detection calibration is met when any of the aboveconditions is met, or a user can set any specific condition from themultiple conditions via the operation unit 121.

TABLE 1 Conditions for starting calibration Setting Condition No.Condition Type 1 Number of printed sheets after the completion of UserSetting the previous line sensor calibration >1000 2 Past time after thecompletion of the previous User Setting line sensor calibration >30minutes 3 Initial operation after line sensor maintenance MachineSetting 4 Initial operation after spectral colorimeter Machinemaintenance Settings101: Any Printing Job?

When the detection calibration condition is not met (Step s100, No), thestate management unit 101 checks if there is a job by receiving printingjobs (Step s101). When receiving printing jobs and there is a printingjob (Step s101, Yes), a printing request is sent to the printing controlunit 103. When there is no printing job (Step s101, No), the procedurereturns to Step s100.

s102: Printing Operation

When the printing request is made at Step s101, the state managementunit 101 sets the operation mode at the normal mode according to theprinting request, and then the printing control unit 103 performsprinting based on the printing job at Step s102. The procedure for theprinting is described later below in detail.

s103: Calibration Operation

When the detection calibration condition is met (Step s100, Yes), adetection-unit calibration request is sent to the calibration unit 109.

At Step s103, the state management unit 101 receives the detection-unitcalibration request and sets the operation mode at the detection-unitcalibration mode. The calibration unit 109 performs calibrationprocesses on the line sensor. The procedure for the detection-unitcalibration is described later below in detail.

The following describes procedure for printing with reference to theflowchart in FIG. 5. The following procedure is performed under thecontrol by the control unit 100.

s200: Obtain Printing Job

Upon the start of the printing process, the printing control unit 103receives a printing request and a printing job from the state managementunit 101 (Step s200).

s201: RIP Process

After obtaining the printing job, the printing control unit 103 performsa RIP process to create a RIP image based on a printing job information(Step s201). Further, a patch can be added to the RIP image for checkingcolor reproducibility. In the RIP image illustrated in FIG. 11, a patchP is arranged at the left end outside of the cropping reference pointsindicated by four crop marks T0 to T3 on the transfer paper S. The patchP is formed having separate areas with different density. In order notto waste the transfer paper, it is preferable that the patch is appliedat a part that is cropped out and thrown away after the fixing.

s202: Printing Instruction

After the RIP process, the printing control unit 103 instructs, as theprinting instruction, the image forming unit 110 to form the RIP imageon the transfer paper and also instructs the paper feeding control unit104 to start feeding the transfer paper (Step s202). The image formingunit 110 controls laser, photoreceptors, the belt, a fixing part and thelike to transfer the image on the transfer paper.

s203: Line Sensor Reading Instruction

After the printing instruction, the state management unit 101 sets theoperation mode at the normal mode and instructs the line sensor controlunit 107 to read the transfer paper with the line sensor 130 (Steps203).

s204: Line Sensor Reading

Based on the reading instruction, the line sensor control unit 107 readsthe image on the transfer paper with the line sensor 130, and then sendsthe image read to the calibration unit 109 via the state management unit101 (Step s204).

s205: Patch Density Calculation

The calibration unit 109 detects the patch from the image read from thetransfer paper with the line sensor 130 and calculates the chromaticityof each patch (Step s205).

s206: Normal Chromaticity?

The calibration unit 109 determines whether the chromaticity is a normalvalue from the detection result of the patch (Step s206). The expectedchromaticity, which is obvious from the image forming condition, iscompared with the detection result so as to determine whether, forexample, the difference in chromaticity is within a standard value(ΔE≤1.0). The standard value can be stored in the storage unit 102 in areadable state or can be set by the user through the operation unit 121.If it is determined as abnormal (Step s206, No), the calibration unit109 sends notification to the state management unit 101 about theabnormal chromaticity.

s207: Next Page?

If it is determined as normal at the determination on the chromaticity(Step s206, Yes), the printing control unit 103 refers to printing jobinformation to check whether there is need for printing a next page,i.e., whether there is a next page (Step s207). If there is a next page(Step s207, Yes), the forming process of the RIP image of the next pageis started (to Step s202). If there is no next page (Step s207, No), thejob is closed.

s208: Cancel Printing

If the detected value is determined as abnormal by the calibration unit109 (Step s206, No), the calibration unit 109 sends a notification tothe state management unit 101 about the abnormal chromaticity and thenthe state management unit 101 cancels the printing (Step s209). Afterthat, the job is closed.

Further, when the printing is cancelled, the state management unit 101can display a notification about the detected abnormal chromaticity onthe operation unit 121 for the user. Further, it is preferable that animage-forming unit calibration button or the like is displayed on theoperation unit 121 so that the user can start the calibration inresponse to the instruction, or that calibration for the image formingunit is automatically performed upon detecting the abnormalchromaticity.

The following describes procedure for the detection-unit calibrationwith reference to the flowchart shown in FIG. 6. The following procedureis performed under control by the control unit 100.

s300: Obtain Calibration Request

When the detection-unit calibration procedure starts, the calibrationunit 109 receives a line sensor calibration request from the statemanagement unit 101 (Step s300).

s301: Create Chart

After obtaining the calibration request (Step s300), the statemanagement unit 101 sends detection-unit calibration chart informationthat is stored in the storage unit 102 to the printing control unit 103.The printing control unit 103 creates a chart image for thedetection-unit calibration (Step s301).

Table 2 shows the detection-unit calibration chart information, whichshows the records of the positions and sizes of the patches. Thedetection-unit calibration chart is illustrated in FIG. 12. Asillustrated in the chart and figure, in the calibration chartinformation, patches 301 to 310 are formed on the transfer paper S inthis order along the sub scanning direction, and there are a pluralityof patches with the same density (301 and 306, 302 and 307, 303 and 308,304 and 309, 305 and 310).

The present invention is not limited to use of the patch images, andnormal images formed on the transfer paper can be used for thecalibration chart without patch images.

TABLE 2 Calibration chart information X Y Patch Position Position WidthHeight C M Y K Quality No. (mm) (mm) (mm) (mm) Density Density DensityDensity Standard 301 130 50 30 30 250 250 250 250 4.50 302 130 80 30 30200 200 200 200 4.75 303 130 110 30 30 150 150 150 150 5.00 304 130 14030 30 100 100 100 100 5.25 305 130 170 30 30 50 50 50 50 5.50 306 130200 30 30 250 250 250 250 4.50 307 130 230 30 30 200 200 200 200 4.75308 130 260 30 30 150 150 150 150 5.00 309 130 280 30 30 100 100 100 1005.25 310 130 310 30 30 50 50 50 50 5.50s302: Printing Instruction

After the creation of the calibration chart (Step s301), the printingcontrol unit 103 instructs the image forming unit 110 to print the chartimage and also instructs the paper feeding control unit 104 to startfeeding the transfer paper (Step s302).

s303: Line Sensor Reading Instruction

After the printing instruction (Step s302), the state management unit101 instructs the line sensor control unit 107 and the colorimetercontrol unit 108 to read the chart image based on the detection-unitcalibration chart information (Step s303). At this process, the statemanagement unit 101 sends a notification to the line sensor control unit107 indicating that the operation mode is set at the detection-unitcalibration mode. Further, the instruction is made to read thebackground board 115A on the conveying path 115 for the reading areabigger than the paper in order to determine the causes of the qualitydefect of the patch. This is because an area outside the paper is usedby the quality determining unit 140 for quality determination describedlater below.

s304: Line Sensor Reading

In response to the line sensor reading instruction, the line sensorcontrol unit 107 reads, via the line sensor 130, the image including thepatch that is formed on the transfer paper by the image forming unit110. Then, the transfer paper image is sent to the storage unit 102 viathe state management unit 101 and is stored therein (Step s304).

s305: Determining Patch Quality

The quality determining unit 140 reads the transfer paper image from thestorage unit 102 to determine printing quality of each patch. The flowof determining quality is described later below.

s306: Colorimetric Instruction for Colorimeter

The state management unit 101 refers to the storage unit 102 andinstructs the colorimeter control unit 108 to read those patches thatare determined as usable for calibration by the patch qualitydetermining unit 140.

In case that the distance of the colorimetry object conveying path 117Abetween the line sensor 130 and the spectral colorimeter 131 is so shortthat the determination by the patch quality determining unit 140 is notmade in time for the transfer paper passing through the spectralcolorimeter 131, control is performed so that the transfer paper can beinserted to the circulating path 118 to pass through the spectralcolorimeter 131 again. Further, the spectral colorimeter 131 can bearranged on the circulating path 118.

s307: Reading Colorimeter

The colorimeter control unit 108 reads, with the spectral colorimeter131, each patch formed on the transfer paper by the image forming unit110. The colorimetric value of patch (L*a*b*/XYZ etc.) is sent to thestorage unit 102 via the state management unit 101. Non-device-dependentL*a*b* or XYZ can be obtained by converting device-dependent RGB valuesor CMYK values of the color space using a profile. The profile can bestored in the storage unit 102.

The calibration chart colorimetric result is illustrated in Table 3.

TABLE 3 Colorimetric result of calibration chart by spectral colorimeterPatch Colorimetric Value No. L* a* b* 301 39.17 3.58 −4.78 303 79.16−9.17 9.84 304 99.74 −2.64 1.28 305 118.31 −4.57 6.52 306 132.33 −5.465.74 307 39.78 −0.76 −6.00 309 77.21 −1.96 4.38 310 100.62 −5.62 3.11s308: Calculating Calibration Value

The calibration unit 109 determines each patch from the transfer paperimage and detects the patch density (RGB/CMYK) (Step s305), calculates acalibration parameter to the line sensor 130 from the patch density andthe colorimetric value of the spectral colorimeter 131, and then storesthe calibration parameter in the storage unit 102. After that, theprocess is closed. When the line sensor 130 performs reading for thenormal image forming calibration, the calibration value can be used forthe calibration of the line sensor 130. Further, in order to check theresult of the calibration, the chart can be read again by the linesensor, the spectral colorimeter or the like to check accuracy of thecalibration. The flow of calculating the calibration value is describedlater below.

s309: Check Needed?

When the operation mode is set at the calibration mode, the statemanagement unit 101 sets the operation mode at the calibration checkmode for checking the calibration accuracy by reading the chart againfor checking the calibration result, and then returns to s301. When theoperation mode is set at the calibration check mode, the calibrationoperation is completed.

The following describes procedure of the line sensor control unit withreference to the flowchart shown in FIG. 7. The following procedure isperformed under control by the control unit 100.

s400: Reading Instruction Received?

The line sensor control unit 107 determines, upon starting its controlprocesses, whether there is a reading instruction of the transfer paperreceived from the state management unit 101 (Step s400). If there is noreading instruction (Step s400, No), the line sensor control unit 107waits until an instruction is made.

s401: Detection-Unit Calibration Mode?

If there is a reading instruction (Step s400, Yes), the line sensorcontrol unit 107 determines, based on the reading instruction receivedfrom the state management unit 101, whether the current operation modeis set at the detection-unit calibration mode (Step s401).

As illustrated in Table 4, the detection-unit calibration parameter maybe or may not be applied depending on the operation mode.

TABLE 4 Calibration parameter setting Operation Calibration ParameterMode Application Normal Yes Calibration Nos402: Skipping Calibration Parameter

In case of the detection-unit calibration mode (Step s401, Yes), theline sensor control unit 107 being set at the detection-unit calibrationmode sets the line sensor 130 at a skip mode where the calibrationparameter is not applied (Step s402).

s403: Applying Calibration Parameter

In case of not being the detection-unit calibration mode (Step s401,No), the line sensor control unit 107 being set at the normal mode readsthe calibration parameter from the storage unit 102 and applies thecalibration parameter on the line sensor 130 (Step s403). An example ofthe calibration parameter is illustrated in Table 5.

TABLE 5 Calibration parameter (RGB LUT) Input R Output R Input G OutputG Input B Output B Value Value Value Value Value Value 0 2 0 1 0 3 1 3 12 1 4 2 5 2 4 2 5 3 7 3 5 3 7 . . . . . . . . . . . . . . . . . . 252243 252 244 252 246 253 245 253 246 253 248 254 246 254 248 254 249 255248 255 250 255 251s404: Reading Preparation

After the Steps s402 and s403, the line sensor control unit 107 startsreading preparation based on the information received by the readinginstruction from the state management unit 101 indicating the transferpaper size, the patch position and the like (Step s404).

s405: Transfer Paper Tip Detected?

After the reading preparation (Step s404), the line sensor control unit107 checks if a line-sensor transfer paper detection sensor 130A is ON(Step s405), and waits until it is ON (Step s405, No). When it is ON(Step s405, Yes), the line sensor control unit 107 determines that thetip of the transfer paper is detected and moves on to the next process(Step s406).

s406: Line Sensor Reading

When the transfer paper tip is detected (Step s405, Yes), the linesensor control unit 107 reads an image line by line at reading intervals(Step s406).

s407: Transfer Paper End Detected?

After the reading (Step s406), the line sensor control unit 107 checksif the line-sensor transfer paper detection sensor 130A is OFF (Steps407, Yes). When it is OFF, the line sensor control unit 107 determinesthat the transfer paper end is detected and moves on to a next process(Step s408). When the transfer paper end detection is not OFF (Steps407, No), the procedure returns to Step s406 and continues the reading.

s408: Reading Completed

When the transfer paper end is detected (Step s407, Yes), the linesensor control unit 107 sends the read image on the transfer paper tothe calibration unit 109 via the state management unit 101 and waits fora reading instruction (to Step s400).

The following describes procedure for colorimeter control with referenceto the flowchart shown in FIG. 8. The following procedure is performedunder control by the control unit 100.

s500: Reading Instruction Received?

The colorimeter control unit 108 determines whether there is a readinginstruction of the transfer paper from the state management unit 101(Step s500). If there is no reading instruction, the colorimeter controlunit 108 waits until there is an instruction (Step s500, No).

s501: Reading Preparation

When there is a reading instruction (Step s500, Yes), the colorimetercontrol unit 108 starts reading preparation based on the informationreceived by the reading instruction from the state management unit 101indicating the transfer paper size, the patch position and the like(Step 501).

s502: Transfer Paper Tip Detected?

After the reading preparation, the colorimeter control unit 108 checksif a colorimetry-object bearing transfer paper detection sensor 131A isON (Step s502). When it is ON (Step s502, Yes), the colorimeter controlunit 108 determines that the tip of the transfer paper is detected andmoves on to the next process. If the transfer paper tip is not detected(Step s502, No), the colorimeter control unit 108 waits until it isdetected.

s503: Reading

When the transfer paper tip is detected (Step s502, Yes), thecolorimeter control unit 108 starts to measure color upon reaching theeach position of the patch instructed by the reading instruction (Steps503).

s504: Transfer Paper End Detected?

The colorimeter control unit 108 checks whether the colorimetry-objectbearing transfer paper detection sensor 131A is OFF. When it is OFF, thecolorimeter control unit 108 determines that the transfer paper end isdetected and moves on to a next process (Step s505). If it is not OFF(Step s504, No), the colorimeter control unit 108 waits until thetransfer paper end is detected.

s505: Reading Completed

After the transfer paper end is detected (Step s504), the colorimetercontrol unit 108 sends the colorimetric value of each patch, specifiedby the reading instruction, to the calibration unit 109 via the statemanagement unit 101 and then waits until there a reading instruction (toStep s500).

The following describes procedure for the quality determining unit 140to determine the quality of the transfer image read by the line sensorwith reference to the flowchart shown in FIG. 9. The following procedureis performed under control by the control unit 100.

s600: Obtaining Line Sensor Image

The quality determining unit 140 obtains the transfer paper image readby the line sensor 130 from the storage unit 102 via the statemanagement unit 101.

s601: Detecting Patch

The quality determining unit 140 detects the patch position on thetransfer paper image, from the chart information stored in the storageunit 102 and the positions of the crop marks on the read image. Thetransfer paper image of the calibration chart read by the line sensor130 is illustrated in FIG. 13.

The transfer paper image includes an image 115AG corresponding to thebackground board 115A on the conveying path 115, images corresponding tothe crop marks T0 to T4 (e.g., a crop mark image T1G), and transferimages 301G to 310G corresponding to the patch images 301 to 310.

S602: Calculating Patch Quality

The quality determining unit 140 performs sampling of the pixel values(RGB/CMYK etc.) of the patch image on the read image based on the patchpositions. For example, the sum of standard deviation of the patch pixelvalues of each color channel is calculated as a patch quality indicatorindicating the margin of error. The pixel values of the patches and thepatch quality indicator are registered in the storage unit 102.

In the example shown in FIG. 13, a plurality of pixel values in thepatch area are sampled for the patch images 301 to 310 (See FIG. 12) onthe transfer paper. The obtained transfer image 302G has unevenprinting, the transfer image 308G has a scratch, and the transfer image310G has line-shape noise. The quality indicator of these three patchesare calculated higher than those of other patches.

Table 6 shows a reading result of the calibration chart. The qualitystandard of each patch is illustrated in Table 2.

TABLE 6 Reading result of calibration chart by line sensor AverageQuality Within Patch Measurement Value Indicator Quality Usable for No.R G B Value Standard Calibration 301 12.2 10.5 14.8 0.85 OK YES 302 52.557.7 49.6 32.91 NG NO 303 101.1 98.4 107.2 1.20 OK YES 304 160.8 154.3155.0 0.98 OK YES 305 212.4 208.6 213.3 1.36 OK YES 306 10.0 11.9 15.71.04 OK YES 307 55.3 51.2 52.4 1.27 OK YES 308 99.8 102.0 106.1 14.73 NGNO 309 159.7 152.1 154.5 1.32 OK YES 310 210.4 206.6 211.9 14.69 NG YESs603: Printing Quality OK?

The quality determining unit 140 determines whether the patch qualityindicator is within the patch quality standard.

s604: Colorimeter Reading Patch Added

When the printing quality is within the quality standard (Step s603,Yes), the quality determining unit 140 registers those patches that havethe patch quality indicator within the patch quality standard as beingusable for the calibration so as to be the colorimeter target by thespectral colorimeter 131.

In FIG. 13, the patches 301, 303 to 307, and 309 are added as thecolorimeter target patches for the spectral colorimeter 131.

s605: Next Patch?

The quality determining unit 140 refers to the chart information whetherthere is a next patch, and, if there is a next patch, moves on to Steps602 to detect the patch. If there is no next patch, the procedure isclosed.

s606: Line Sensor Stained?

When the patch quality indicator is not within the quality standard atthe quality determining unit 140 (Step s603, No), it is determinedwhether the cause is printing of the chart or a stain of the line sensor130.

The determination process is made on the patches 302, 308 and 310 thatare determined as being out of the patch quality standard. When theline-shape noise that continues along the paper feeding directionfurther continues to the background board image 115AG, like on thetransfer image 310G, it is determined that the cause is a dust attachedto the line sensor 130. When the cause is not a stain on the line sensor(Step s606, No), the procedure moves on to Step s605 to determinewhether there is a next patch.

s607: Usable?

The quality determining unit 140 determines whether those patch imageswith defective image due to a stain of the line sensor 130 (Step s606,Yes) are usable for calibration or not.

The area with the stain on the patch is considered as a defective area.When the size of the defective area is the predetermined value (forexample, below 30%) of the patch size, the pixel values outside thedefective area are sampled again and updated on the storage unit 102 asthe colorimetric values. Further, the patch is registered as beingusable for the calibration by the spectral colorimeter 131 and set astargets to be measured by the spectral colorimeter 131. Thepredetermined value above is previously set and stored in the storageunit 102 and the like, or can be set by a user through the operationunit 121. When the patch image with a defective image is detected, auser can determine if the patch image is usable or not through theoperation unit 121.

As for the transfer image 310G of the patch 310, which is defective dueto a stain of the line sensor, the pixel values of the area excludingthe line-shape noise are sampled again to be updated as the measurementvalues of the patch. The patch 310 is added as the colorimeter targetpatch for the spectral colorimeter 131. In conclusion, if usable (Steps607, Yes), the procedure moves on to Step s604 and selects thecolorimeter reading patch, and if not usable (Step s607, No), theprocedure moves on to Step s605 and determines whether there is a nextpatch.

As for those patches that are determined as defective due to the chartprinting at Step s606, if the deficiency is small, the image can beprocessed and the area excluding the defective area can be colorimetertargets for the spectral colorimeter 131. For example, when there is ascratch or a line at the end of the patch, the color of the center ofthe patch can be measured by the spectral colorimeter 131 so that theeffect of the deficiency can be avoided.

The following describes procedure for calculating the calibration valuefor the line sensor 130 based on the detection result of the image bythe line sensor 130 and the spectral colorimeter 131 with reference tothe flowchart shown in FIG. 10. The following procedure is performedunder control by the control unit 100.

s700: Obtaining Line Sensor Reading Value

The calibration unit 109 obtains a measurement value of the line sensor130 from the storage unit 102 via the state management unit 101.

s701: Obtaining Colorimetric Value of Spectral Colorimeter

The calibration unit 109 obtains the colorimetric value of the spectralcolorimeter 131 from the storage unit 102 via the state management unit101.

s702: Check Calibration?

The calibration unit 109 determines whether the operation mode is set atthe calibration check mode. In case of the calibration check mode (Steps702, Yes), a check process is performed to check whether the differenceof the measurement value of the line sensor 130 and the colorimetricvalue of the spectral colorimeter 131 is predetermined value (to Steps705). In case of the calibration mode (Step s702, No), a process tocreate a calibration parameter for the line sensor 130 is performedbased on the measurement value of the line sensor 130 and thecolorimetric value of the spectral colorimeter 131 (to Step s703).

s703: Calibration Possible?

The calibration unit 109 determines whether calibration can be made atsufficient accuracy based on the colorimetric data measured by the linesensor 130 and the spectral colorimeter 131. When the colorimetricvalues of patches of all densities are given or when the datacorresponding to a specific density is lacking yet can be substituted byestimating from data of other densities, it is determined that thecalibration is possible (Step s703, Yes) and a calibration parameter iscreated (to Step s704). The method of substitution is previouslydetermined. If the calibration is not possible (Step s703, No), anotification indicating the failure of calibration is displayed (Steps706).

Out of a total of ten patches with five kinds of densities, the patches301, 303 to 307, 309 and 310 measured by the spectral colorimeter 131cover all five kinds of densities, and therefore it is determined thatthe calibration is possible.

s704: Creating Calibration Parameter

The calibration unit 109 calculates a calibration parameter of the linesensor 130 based on colorimetric data measured by the line sensor 130and the spectral colorimeter 131.

s705: Normal Color Difference?

The calibration unit 109 calculates the color difference between themeasurement value of the line sensor 130 and the colorimetric value ofthe spectral colorimeter 131 and determines whether color difference iswithin a predetermined value (for example, ΔE≤1.0). If the colordifference is normal (Step s705, Yes), a notification indicating thesuccess of calibration is displayed (Step s707). If the color differenceis not normal (Step s705, No), a notification indicating the failure ofcalibration is displayed (Step s706).

s706: Calibration Failure Display

The calibration unit 109 notifies a user by displaying a notificationindicating that the calibration is not possible on the operation unit121.

It is preferable that a notification is displayed for encouraging a userto clean the line sensor or adjust the image forming unit, or thatre-calibration button is displayed for encouraging the user to operatethe re-calibration.

s707: Calibration Success Display

The calibration unit 109 notifies a user by displaying a notificationindicating the success of calibration on the operation unit 121.

After the Steps s704, s706, and s707, the process is closed.

Second Embodiment

In the previous embodiment, the patch image is formed in a single linealong the sub scanning direction corresponding to the readable area ofthe spectral colorimeter 131. In this embodiment, the spectralcolorimeter 131 is configured to be movable along the main scanningdirection so that a different patch image created along the mainscanning direction is also readable. The same reference number is usedfor the same configuration as the previous embodiment, and thedescription thereof is omitted or simplified.

In an image forming system 1A according to the present embodiment, thespectral colorimeter 131 is supported by a colorimeter moving unit 132on the conveying path 115 as shown in FIG. 14. The colorimeter movingunit 132 can be moved along the main scanning direction with use of adriving unit such as a driving motor, and the movement is controlled bythe control unit 100. The configuration of the colorimeter moving unit132 is not limited to this particular configuration.

As shown in FIG. 15, the spectral colorimeter 131 is supported by thecolorimeter moving unit 132, and thus the spectral colorimeter 131 canbe moved along the main scanning direction by the colorimeter movingunit 132. As shown in the figure, a plurality of patch images having thesame densities are formed along the main scanning direction and alsopatch images on a plurality of lines are formed along the sub scanningdirection. The spectral colorimeter 131 can measure colors of thepredetermined area, and, because the spectral colorimeter 131 is movableby the colorimeter moving unit 132, the spectral colorimeter 131 canalso read images at different positions along the main scanningdirection. As a result, the spectral colorimeter can avoid using adetection result of patch images on a line with defective images, andinstead use only the suitable reading images to perform the colormeasurement.

As for the patch image, the patch image having the same density isformed along the main scanning direction as shown in FIG. 16. Theinformation on the patch image is shown in Table 7.

The patch images 321 to 323 have the same density, the patch images 324to 326 have the same density, and the patch images 327 to 329 have thesame density. The patch images 321 to 323, the patch images 324 to 326,and the patch images 327 to 329 are formed in different image densities.As for the scale of the density along a line, the patch image 321 hashigher density than the patch image 324, and the patch image 324 hashigher density than the patch image 327. With these patch images, thereading result of the image with image deficiency may not be used, andthe reading result of other images having the same density can be usedinstead.

TABLE 7 Calibration chart information X Y Patch Position Position WidthHeight C M Y K Quality No. (mm) (mm) (mm) (mm) Density Density DensityDensity Standard 321 40 50 30 30 250 250 250 250 4.00 322 120 50 30 30150 150 150 150 5.00 323 200 50 30 30 50 50 50 50 5.50 324 40 150 30 30250 250 250 250 4.00 325 120 150 30 30 150 150 150 150 5.00 326 200 15030 30 50 50 50 50 5.50 327 40 250 30 30 250 250 250 250 4.00 328 120 25030 30 150 150 150 150 5.00 329 200 250 30 30 50 50 50 50 5.50

The following describes a quality determination of an image andcolorimeter control. The contents of the procedure are the same as theflows shown in FIG. 8 and FIG. 9.

s600: Obtaining Line Sensor Image

The quality determining unit 140 obtains the transfer paper image readby the line sensor 130 from the storage unit 102 via the statemanagement unit 101.

s601: Detecting Patch

The quality determining unit 140 detects the patch position on thetransfer paper image, based on the chart information stored in thestorage unit 102 and the crop mark position on the read image. Thetransfer paper image obtained by the line sensor 130 reading thecalibration chart is illustrated in FIG. 17. On the transfer paperimage, a background image 151AG and transfer images 321G to 329G aredetected based on the patch images.

s602: Calculating Patch Quality

The quality determining unit 140 samples pixel values (RGB/CMYK etc.) ofthe patch image on the read image based on the patch position. Forexample, the sum of the standard deviation of the patch pixel values ofeach color channel is calculated as a patch quality indicator indicatingmargin of error. The patch pixel values and the patch quality indicatorare stored in the storage unit 102.

In an example shown in FIG. 17, a plurality of pixel values in the patcharea are sampled for these patches 321 to 329 (see FIG. 16) of thetransfer paper image. The obtained transfer image 323G has unevenprinting, the transfer image 329G has a scratch, and the transfer images321G, 324G and 327G have line-shape noise. The quality indicators ofthese three patches are calculated higher than those of other patches.

The reading results of the calibration chart are shown in Table 8 andthe quality standard of each patch is shown in Table 8.

TABLE 8 Reading result of calibration chart by line sensor AverageQuality Within Patch Measurement Value Indicator Quality Usable for No.R G B Value Standard Calibration 321 12.3 10.8 14.5 5.95 NG YES 322101.9 98.6 107.2 0.85 OK YES 323 212.0 208.2 213.1 34.06 NG NO 324 9.313.7 12.7 6.07 NG YES 325 103.8 97.4 105.9 0.89 OK YES 326 211.4 209.3210.2 1.34 OK YES 327 11.2 11.5 15.8 7.21 NG YES 328 103.6 100.1 104.30.76 OK YES 329 213.7 207.0 209.4 15.78 NG NOs603: Printing Quality OK?

The quality determining unit 140 determines whether the patch qualityindicator is within the quality standard of the patch.

s604: Colorimeter Reading Patch Added

When the printing quality is within the quality standard (Step s603,Yes), the quality determining unit 140 registers those patches that havethe patch quality indicator within the patch quality standard as beingusable for the calibration so as to be the colorimeter target for thespectral colorimeter 131.

In FIG. 17, the patches 322, 325, 326 and 329 are added as thecolorimeter target patches for the spectral colorimeter 131.

s605: Next Patch?

The quality determining unit 140 refers to the chart information whetherthere is a next patch, and, if there is a next patch, moves on to Steps602 to detect the patch. If there is no next patch, the procedure isclosed.

s606: Line Sensor Stained?

When the patch quality indicator is not within the quality standard atthe quality determining unit 140, it is determined whether the cause isprinting of the chart or a stain of the line sensor 130.

The determination process is made on the patches 321, 323, 324, 327 and328 that are determined as being out of the patch quality standard. Whenthe line-shape noise that continues along the paper feeding directionfurther continues to the background board image 151AG, like on thetransfer images 321G, 324G and 327G, it is determined that the cause isa dust attached to the line sensor 130. When the cause is not a stain onthe line sensor (Step s606, No), the procedure moves on to Step s605 todetermine whether there is a next patch.

s607: Usable?

The quality determining unit 140 determines whether the those imagepatches with defective images due to the stain of the line sensor 130are usable for calibration or not.

The area with the stain on the patch is considered as a defective area.When the size of the defective area is the predetermined value (forexample, below 30%) of the patch size, the pixel values outside thedefective area are sampled again and updated on the storage unit 102 asthe colorimetric values. Further, the patch is registered as beingusable for the calibration by the spectral colorimeter 131 and set astargets to be measured by the spectral colorimeter 131. Thepredetermined value above is previously set and stored in the storageunit 102 and the like, or can be set by a user through the operationunit 121. When the patch image with a defective image is detected, auser can determine if the patch image is usable or not through theoperation unit 121.

As for the transfer images 321G, 324G and 327G of the patches 321, 324and 327, which are defective due to a stain of the line sensor, thepixel values of the area excluding the line-shape noise are sampledagain to be updated as the measurement values of the patch. The patches321, 324 and 327 are added as the colorimeter target patches for thespectral colorimeter 131.

As for those patches that are determined as defective due to the chartprinting at Step s606, if the deficiency is small, the image can beprocessed and the area excluding the defective area can be colorimetertargets for the spectral colorimeter 131. For example, when there is ascratch or a line at the end of the patch, the color of the center ofthe patch can be measured by the spectral colorimeter 131 so that theeffect of the deficiency can be avoided.

s500: Reading Instruction Received?

The colorimeter control unit 108 determines whether there is a readinginstruction of the transfer paper from the state management unit 101(Step s500). If there is no reading instruction, the colorimeter controlunit 108 waits until there is an instruction (Step s500, No).

s501: Reading Preparation

When there is a reading instruction (Step s500, Yes), the colorimetercontrol unit 108 starts reading preparation based on the informationreceived by the reading instruction from the state management unit 101indicating the transfer paper size, the patch position and the like. Atthis process, the colorimeter control unit 108 moves the spectralcolorimeter 131 from the base position to the first patch position usingthe colorimeter moving unit 132.

The color measurement is prepared for those patches 321, 322, 324 to327, and 329 in FIG. 16 that are determined as colorimeter targets bythe quality determining unit 140.

s502: Transfer Paper Tip Detected?

It is checked whether the colorimetry-object bearing transfer paperdetection sensor 131A is ON (Step s502). When it is ON (Step s502, Yes),it is determined that the tip of the transfer paper is detected and theprocedure moves on to the next process. If the transfer paper tip is notdetected (Step s502, No), the colorimeter control unit 108 waits untilit is detected.

s503: Reading

After the reading preparation, the colorimeter control unit 108 moves,using the colorimeter moving unit 132, the spectral colorimeter 131 toeach patch position specified by the reading instruction along the crossdirection of the paper feeding direction to start the color measurement.

The movement of the spectral colorimeter 131 using the colorimetermoving unit 132 can be carried out once for each chart, and in this way,only the patches 321, 324, and 327 arranged in the paper feedingdirection can be read. For example, in order to measure the color of thepatches 321 and 322 arranged in the cross direction to the paper feedingdirection, the conveying of the chart needs to be temporarily stoppedwhile the spectral colorimeter is moved, which takes a lot of time forcolor measurement. Thus, in the way above, this time can be saved.

s504: Transfer Paper End Detected?

The colorimeter control unit 108 checks whether the colorimetry-objectbearing transfer paper detection sensor 131A is OFF. When it is OFF, thecolorimeter control unit 108 determines that the transfer paper end isdetected and moves on to a next process (Step s505). If it is not OFF(Step s504, No), the colorimeter control unit 108 waits until thetransfer paper end is detected.

s505: Reading Complete

After the transfer paper end is detected (Step s504), the colorimetercontrol unit 108 sends the colorimetric value of each patch, specifiedby the reading instruction, to the calibration unit 109 via the statemanagement unit 101. Then, the colorimeter control unit 108 moves thespectral colorimeter 131 to the base position using the colorimetermoving unit 132 and waits until there a reading instruction (to Steps500).

To prevent a stain due to a paper dust or the like, it is preferablethat the base position is arranged outside the colorimetric path 117A.

The detection result from the spectral colorimeter 131 is shown in Table9.

TABLE 9 Colorimetric result of calibration chart by spectral colorimeterPatch Colorimetric Value No. L* a* b* 321 39.17 3.58 −4.78 322 99.74−2.64 1.28 323 132.33 −5.46 5.74 324 41.30 −9.04 2.73 325 99.56 −1.822.12 326 132.39 −6.33 6.84 327 40.14 0.59 −5.43 328 100.34 −3.63 3.83329 132.14 −5.38 6.82

In this embodiment, the spectral colorimeter 131 is moved along the mainscanning direction to allow to detect the patch images at differentpositions. Instead, the transfer paper can be moved along the mainscanning direction to allow to detect the patch images at differentpositions. Both the spectral colorimeter 131 and the transfer paper canbe movable along the main scanning direction.

Third Embodiment

Although the embodiments above describe reading of a patch image printedon a single side of the transfer paper, the reading of the image printedon the back side thereof can be also carried out.

FIG. 18 shows an image forming system 1B where the reverse path 119 isprovided at the circulating path 118. Although not shown in the figure,with the circulating path 118, the transfer paper whose front and backare reversed through the reverse path 119 can be returned to theconveying path 115 at the upper stream side of the line sensor 130. Theprocess is the same as the previous embodiment, and the descriptionthereof is omitted or simplified with the same reference numbersattached.

The processing procedures for the overall flow, printing flow, andquality determining flow are the same as the first embodiment, and thereis difference only in the calibration flow. Therefore, the followingdescribes procedure for the calibration flow with reference to theflowchart shown in FIG. 19. The following procedure is performed undercontrol by the control unit 100.

s1000: Obtaining Calibration Request

When the detection-unit calibration procedure starts, the calibrationunit 109 receives a line sensor calibration request from the statemanagement unit 101.

s1001: Chart Creation

After obtaining the calibration request (Step s1000), the statemanagement unit 101 sends the detection-unit calibration chartinformation stored in the storage unit 102 to the printing control unit103. The printing control unit 103 creates a chart image for thedetection-unit calibration (Step s1001).

In order to make the printing on the back side of the transfer paper,the front and back sides of the paper are reversed through the reverseconveying path 116 after the fixing of the image, and the paper iscirculated to the conveying path 115 at the upper stream side of theimage forming unit 110. In order to form the patch image on the backside of the transfer paper, it is preferable that a chart is created forforming the back-side patch image at the positions where the front-sidepatch image is not formed so that the colorimetric value is not affectedby the offset.

s1002: Printing Instruction

After the calibration chart is created (Step s1001), the printingcontrol unit 103 instructs the image forming unit 110 to print the chartimage and instructs the paper feeding control unit 104 to start feedingthe transfer paper. In order to form the patch image on the back side,the front and back sides of the paper are reversed on the reverseconveying path 119 after the chart image is formed on the front side ofthe transfer paper under the control of the conveying control unit 106,and then the transfer paper is conveyed to the conveying path 115 at theupper stream side of the image forming unit 110.

s1003: Line Sensor Reading Instruction

Based on the detection-unit calibration chart information, the statemanagement unit 101 instructs the line sensor control unit 107 and thecolorimeter control unit 108 to read the chart image. At this process,the state management unit 101 sends a notification to the line sensorcontrol unit 107 indicating that the operation mode is set at thedetection-unit calibration mode. Further, the instruction is made toread the background board 115A on the conveying path 115 for the readingarea bigger than the paper in order to determine the causes of thequality defect of the patch. This is because an area outside the paperis used by the quality determining unit 140 for quality determinationdescribed later below.

s1004: Line Sensor Reading

In response to the line sensor reading instruction, the line sensorcontrol unit 107 reads, via the line sensor 130, the image including thepatch that is formed on the transfer paper by the image forming unit110. Then, the transfer paper image is sent to the storage unit 102 viathe state management unit 101 and is stored therein.

s1005: Determining Patch Quality

The quality determining unit 140 reads the transfer paper image from thestorage unit 102 to determine printing quality of each patch. The flowof determining printing quality is the same as the procedure shown inFIG. 9.

s1006: Colorimetric Instruction for Colorimeter

The state management unit 101 refers to the storage unit 102 andinstructs the colorimeter control unit 108 to start the reading process.

In case that the distance of the colorimetry object conveying path 117Abetween the line sensor 130 and the spectral colorimeter 131 is so shortthat the determination by the patch quality determining unit 140 is notmade in time for the transfer paper passing through the spectralcolorimeter 131, control is performed so that the transfer paper can beinserted to the circulating path 118 to pass through the spectralcolorimeter 131 again.

s1007: Reading Colorimeter

The colorimeter control unit 108 reads, with the spectral colorimeter131, each patch formed on the transfer paper by the image forming unit110. The colorimetric value of patch (L*a*b*/XYZ etc.) is sent to thestorage unit 102 via the state management unit 101. Non-device-dependentL*a*b* or XYZ can be obtained by converting device-dependent RGB valuesor CMYK values of the color space using a profile. The profile can bestored in the storage unit 102.

s1008: Back Side Needed?

When the state management unit 101 refers to the storage unit 102 andthere are those patches that are determined as being out of the qualitystandard by the quality determining unit 140, it is determined thatreading of the patch on the back side is needed, and the transfer paperis reversed through the reverse path 119 and then returned to thecolorimetric path 117A. Thus, even when the printing quality of thepatch on the front side is out of the standard, the measurement valuesneeded for the calibration can be obtained by reading the patch on theback side.

S1009: Calculating Calibration Value

The calibration unit 109 determines each patch from the transfer paperimage and detects the patch density (RGB/CMYK) (Step s1005), calculatesa calibration parameter to the line sensor 130 from the patch densityand the colorimetric value of the spectral colorimeter 131, and thenstores the calibration parameter in the storage unit 102. After that,the process is closed. When the line sensor 130 performs reading for thenormal image forming calibration, the calibration value can be used forthe calibration of the line sensor 130.

s1010: Check Needed?

When the operation mode is set at the calibration mode, the statemanagement unit 101 sets the operation mode at the calibration checkmode for checking the calibration accuracy by reading the chart againfor checking the calibration result, and then returns to s301. When theoperation mode is set at the calibration check mode, the calibrationoperation is completed.

Fourth Embodiment

When quality of the image read by the line sensor is higher than orequal to the predetermined quality in the embodiments above, the imageis read by the spectral colorimeter. However, the image read by the linesensor can be read by the spectral colorimeter, and the calibration canbe performed using only the detection result indicating that quality ofthe image read by the line sensor is higher than or equal to thepredetermined quality. In the procedure below, the flows are the same asthe first embodiment except the calibration flow and the qualitydetermining flow, and therefore only the calibration flow and thequality determining flow are described below.

First, the following describes procedure of the calibration flow withreference to the flowchart shown in FIG. 20. The following procedure isperformed under control by the control unit 100.

s1100: Obtaining Calibration Request

When the detection-unit calibration procedure starts, the calibrationunit 109 receives a line sensor calibration request from the statemanagement unit 101.

s1101: Chart Creation

After obtaining the calibration request (Step s1100), the statemanagement unit 101 sends the detection-unit calibration chartinformation stored in the storage unit 102 to the printing control unit103. The printing control unit 103 creates a chart image for thedetection-unit calibration (Step s1101).

s1102: Printing Instruction

After the creation of the calibration chart (Step s1101), the printingcontrol unit 103 instructs the image forming unit 110 to print the chartimage and also instructs the paper feeding control unit 104 to startfeeding the transfer paper.

s1103: Line Sensor Reading Instruction

Based on the detection-unit calibration chart information, the statemanagement unit 101 instructs the line sensor control unit 107 and thecolorimeter control unit 108 to read the chart image. At this process,the state management unit 101 sends a notification to the line sensorcontrol unit 107 indicating that the operation mode is set at thedetection-unit calibration mode. Further, the instruction is made toread the background board 115A on the conveying path 115 for the readingarea bigger than the paper in order to determine the causes of thedefect of the patch. This is because an area outside the paper is usedby the quality determining unit 140 for quality determination describedlater below.

s1104: Line Sensor Reading

In response to the line sensor reading instruction, the line sensorcontrol unit 107 reads, via the line sensor 130, the image including thepatch that is formed on the transfer paper by the image forming unit110. Then, the transfer paper image is sent to the storage unit 102 viathe state management unit 101 and is stored therein.

s1105: Colorimetric Instruction for Colorimeter

The state management unit 101 refers to the storage unit 102 andinstructs the colorimeter control unit 108 to start the reading process.

In case that the distance of the colorimetry object conveying path 117Abetween the line sensor 130 and the spectral colorimeter 131 is so shortthat the determination by the quality determining unit 140 is not madein time for the transfer paper passing through the spectral colorimeter131, control is performed so that the transfer paper can be to thecirculating path 118 to pass through the spectral colorimeter 131 again.

s1106: Reading Colorimeter

The colorimeter control unit 108 reads, with the spectral colorimeter131, each patch formed on the transfer paper by the image forming unit110. The colorimetric value of patch (L*a*b*/XYZ etc.) is sent to thestorage unit 102 via the state management unit 101. Non-device-dependentL*a*b* or XYZ can be obtained by converting device-dependent RGB valuesor CMYK values of the color space using a profile. The profile can bestored in the storage unit 102.

s1107: Determining Patch Quality

The quality determining unit 140 reads the transfer paper image from thestorage unit 102 to determine printing quality of each patch. The flowof determining printing quality is the same as the procedure shown inFIG. 9.

s1108: Calculating Calibration Value

The calibration unit 109 determines each patch from the transfer paperimage and detects the patch density (RGB/CMYK) (Step s1107), calculatesa calibration parameter to the line sensor 130 from the patch densityand the colorimetric value of the spectral colorimeter 131 for thetransfer image of which patch quality is appropriate, and then storesthe calibration parameter in the storage unit 102. After that, theprocess is closed. When the line sensor 130 performs reading for thenormal image forming calibration, the calibration value can be used forthe calibration of the line sensor 130.

s1109: Check Needed?

When the operation mode is set at the calibration mode, the statemanagement unit 101 sets the operation mode at the calibration checkmode for checking the calibration accuracy by reading the chart againfor checking the calibration result, and then returns to s1101. When theoperation mode is set at the calibration check mode, the calibrationoperation is completed.

The following describes procedure of the quality determining flow withreference to the flowchart shown in FIG. 21. The following procedure isperformed under control by the control unit 100.

s1200: Obtaining Line Sensor Image

The quality determining unit 140 obtains the transfer paper image readby the line sensor 130 from the storage unit 102 via the statemanagement unit 101.

s1201: Detecting Patch

The quality determining unit 140 detects the patch position on thetransfer paper image, based on the chart information stored in thestorage unit 102 and the crop mark position on the read image.

The transfer paper image includes an image 115AG corresponding to thebackground board 115A on the conveying path 115 and the transfer images321G to 329G corresponding to the patch images 321 to 329.

S1202: Calculating Patch Quality

The quality determining unit 140 samples pixel values (RGB/CMYK etc.) ofthe patch image on the read image based on the patch position. Forexample, the sum of the standard deviation of the patch pixel values ofeach color channel is calculated as a patch quality indicator indicatingmargin of error. The patch pixel values and the patch quality indicatorare stored in the storage unit 102.

s1203: Printing Quality OK?

The quality determining unit 140 determines whether the patch qualityindicator is within the patch quality standard.

s1204: Selecting Patch for Calibration

When the printing quality is within the quality standard (Step s1203,Yes), the quality determining unit 140 registers those patches that havethe patch quality indicator within the patch quality standard as beingusable for the calibration.

S1205: Next Patch?

The quality determining unit 140 refers to the chart information whetherthere is a next patch, and, if there is a next patch, moves on to Steps1202 to detect the patch. If there is no next patch, the procedure isclosed.

s1206: Line Sensor Stained?

When the patch quality indicator is not within the quality standard atthe quality determining unit 140 (Step s1203, No), it is determinedwhether the cause is printing of the chart or a stain of the line sensor130.

The determination process is made on those patches that are determinedas being out of the patch quality standard. When the cause is not astain on the line sensor (Step s1206, No), the procedure moves on toStep s1205 to determine whether there is a next patch.

s1207: Usable?

The quality determining unit 140 determines whether the those patchimages with defective images due to the stain of the line sensor 130 areusable for calibration or not.

The area with the stain on the patch is considered as a defective area.When the size of the defective area is the predetermined value (forexample, below 30%) of the patch size, the pixel values outside thedefective area are sampled again and updated on the storage unit 102 asthe colorimetric values. Further, the patch is registered as beingusable for the calibration by the spectral colorimeter 131. Thepredetermined value above is previously set and stored in the storageunit 102 and the like, or can be set by a user through the operationunit 121. When the patch image with a defective image is detected, auser can determine if the patch image is usable or not through theoperation unit 121.

As for the transfer images 321G, 324G and 327G of the patch images 321,324 and 327, which are defective due to a stain of the line sensor, thepixel values of the area excluding the line-shape noise are sampledagain to be updated as the measurement values of the patch.

As for those patches that are determined as defective due to the chartprinting at Step s1206, if the deficiency is small, the image can beprocessed and the area excluding the defective area can be colorimetertargets for the spectral colorimeter 131. For example, when there is ascratch or a line at the end of the patch, the color of the center ofthe patch can be measured by the spectral colorimeter 131 so that thedeficiency can be avoided.

In this embodiment, the detection result from the second image detectionunit is used depending on the quality result of the image read by thefirst image detection unit. Thus, there is an advantage that thecalibration of the first image detection unit can be performed reliablyand accurately.

The present invention is described in accordance with the embodimentsabove. However, the present invention can be modified arbitrarilywithout departing from the scope of the present invention.

What is claimed is:
 1. An image forming system comprising: an imageforming unit that forms an image on a recording medium; a conveying unitthat conveys the recording medium through a conveying path; a linesensor that reads the image formed by the image forming unit on therecording medium conveyed by the conveying unit; a colorimeter thatmeasures the image on the recording medium and is provided at thedownstream side of the line sensor; and a control unit that receives acolorimetric result from the colorimeter and a reading result from theline sensor, wherein the control unit performs calibrations of the linesensor based on the colorimetric result from the colorimeter.
 2. Theimage forming system of claim 1, wherein the line sensor comprises acharged coupled device (CCD) or comprises contact image sensors (CIS).3. The image forming system of claim 1, wherein the line sensor coversthe recording medium at a first region having a first width, and thecolorimeter covers the recording medium at a second region having asecond width that is less than the first width.
 4. The image formingsystem of claim 1, further comprising a circulating path, wherein thecirculating path diverges from the conveying path to allow the recordingmedium, with a first side of the recording medium facing up, to beconveyed from the conveying path to the circulating path, and thecirculating path converges to the conveying path at an upstream side ofthe line sensor to allow the recording medium to be returned from thecirculating path to the conveying path at a location upstream of theline sensor.
 5. The image forming system of claim 4, wherein thecirculating path diverges from the conveying path at a downstream sideof the colorimeter.
 6. The image forming system of claim 4, furthercomprising a reverse path controlled by the control unit and configuredto reverse sides of the recording medium to allow the recording mediumto be returned from the circulating path to the conveying path at thelocation upstream of the line sensor with a second side of the recordingmedium facing up.
 7. The image forming system of claim 1, wherein theline sensor and the colorimeter are provided in the image forming unit,and the image forming unit is provided in an image forming apparatus. 8.The image forming system of claim 1, further comprising an image formingapparatus; and a post-processing apparatus connected to a downstreamside of the image forming apparatus, wherein the image forming apparatuscomprises the image forming unit, the line sensor is provided in theimage forming unit, and the colorimeter is provided in thepost-processing apparatus.
 9. The image forming system of claim 1,further comprising an image forming apparatus; and a post-processingapparatus connected to a downstream side of the image forming apparatus,wherein the image forming apparatus comprises the image forming unit,and the line sensor and the colorimeter are provided in thepost-processing apparatus.
 10. The image forming system of claim 1,wherein the calorimeter is a spectral colorimeter.
 11. The image formingsystem of claim 1, wherein one or more rollers are located downstream ofthe line sensor and upstream of the colorimeter.
 12. The image formingsystem of claim 1, wherein the image forming unit is a fixing deviceconfigured to form the image on the recording medium via heat-fixing.